Many of the red-emitting phosphors are derived from silicon nitride (Si3N4). The structure of silicon nitride comprises layers of Si and N bonded in a framework of slightly distorted SiN4 tetrahedra. The SiN4 tetrahedra are joined by a sharing of nitrogen corners such that each nitrogen is common to three tetrahedra. See, for example, S. Hampshire in “Silicon nitride ceramics—review of structure, processing, and properties,” Journal of Achievements in Materials and Manufacturing Engineering, Volume 24, Issue 1, September (2007), pp. 43-50. Compositions of red-emitting phosphors based on silicon nitride often involve substitution of the Si at the center of the SiN4 tetrahedra by elements such as Al; this is done primarily to modify the optical properties of the phosphors, such as the intensity of the emission, and the peak emission wavelength.
There is a consequence of the aluminum substitution, however, which is that since Si4+ is being replaced by Al3+, the substituted compound develops a missing positive charge. There are essentially two ways commonly employed to achieve charge balance: in one scheme, an Al3+ for Si4+ substitution is accompanied by a substitution of O2− for N3−, such that the missing positive charge is counter-balanced with a missing negative charge. This leads to a network of tetrahedra that have either Al3+ or Si4+ as the cations at the centers of the tetrahedra, and a structure whereby either an O2− or an N3− anion is at the corners of the tetrahedra. Since it is not known precisely which tetrahedra have which substitutions, the nomenclature used to describe this situation is (Al,Si)3—(N,O)4. Clearly, for charge balance there is one O for N substitution for each Al for Si substitution.
Furthermore, these substitutional mechanisms for charge balance—O for N—may be employed in conjunction with an interstitial insertion of a cation. In other words, the modifying cation is inserted between atoms preexisting on crystal lattice sites, into “naturally occurring” holes, interstices, or channels. This mechanism does not require an altering of the anionic structure (in other words, a substitution of O for N), but this is not to say that an O for N substitution may not simultaneously occur. Substitutional mechanisms for charge balance may occur in conjunction with an interstitial insertion of a modifier cation.
The use of modifying cations in nitride phosphors of Sr-containing α-SiAlON has been discussed by K. Shioi et al. in “Synthesis, crystal structure, and photoluminescence of Sr-α-SiAlON:Eu2+,” J. Am. Ceram Soc., 93 [2] 465-469 (2010). Shioi et al. give the formula for the overall composition of this class of phosphors: Mm/vSi12−m−nAlm+nOnN16−n:Eu2+, where M is a “modifying cation” such as Li, Mg, Ca, Y, and rare earths (excluding La, Ce, Pr, and Eu), and v is the valence of the M cation. As taught by Shioi et al., the crystal structure of an α-SiAlON is derived from the compound α-Si3N4. To generate an α-SiAlON from α-Si3N4, a partial replacement of Si4+ ions by Al3+ ions takes place, and to compensate for the charge imbalance created by Al3+ substituting for Si4+, some O substitutes N and some positive charges are added by trapping the M cations into the interstices within the network of (Si,Al)—(O,N)4 tetrahedra.
Europium doped alkaline earth metal silicon nitride phosphor with the general formula M2Si5N8:Eu, where M is Ca, Sr, or Ba, have been widely studied, see for example the PhD thesis by JWH van Krevel at the Technical University Eindhoven, January 2000, and H. A. Hoppe, et al., J. Phys. Chem. Solids. 2000, 61:2001-2006. In this family of phosphors, pure Sr2Si5N8:Eu has high quantum efficiency and emits at a peak wavelength of about 620 nm. However, this red nitride phosphor has poor stability when used as a coating on an LED operated at a temperature in the range from 60° C. to 120° C. and an ambient relative humidity in the range from 40% to 90%.
Various research groups have experimented with oxygen-containing M2Si5N8 based phosphor materials, which may also contain other metals. For example, see U.S. Pat. Nos. 7,671,529 and 6,956,247, and U.S. published applications 2010/0288972, 2008/0081011, and 2008/0001126. However, these oxygen containing phosphor materials are known to exhibit poor stability under the combined conditions of high temperature and high relative humidity (RH)—for example 85° C. and 85% RH.
The forms of charge compensation reported in the art are not believed to render the phosphor more impervious to thermal/humidity aging, nor do they appear to accomplish the beneficial result of increasing the peak emission wavelength with little or substantially no alteration of photoemission intensity.
There is a need for silicon nitride-based phosphors and M2Si5N8-based phosphors with peak emission wavelengths over a wider range in the red and also other colors, and with enhanced physical properties of the phosphor, such as temperature and humidity stability.